1. Field of the Invention
The present invention relates to the fabrication of refractive microlenses. More specifically, a method of forming microlenses self-aligned on a resin pillar is proposed.
2. Description of the Related Technology
Microlenses are microscopic passive optical components that fit on active optoelectronic devices such as detectors, displays, and light emitting devices (light-emitting diodes, transversal and vertical cavity lasers) to improve their optical input or output quality. The areas of applications are wide and cover areas such as telecommunications, information technology, audio-visual services, solar cells, detectors, solid-state light sources, and optical interconnects.
There are basically two types of micro-lenses: diffractive lenses and refractive lenses. A diffractive microlens deflects a light beam by diffraction. It typically consists of single-level or multilevel concentric rings sculptured in a material with refractive index different from the environment.
A refractive microlens deflects a light beam by refraction at its curved surfaces. It is mostly plano-convex: one surface of the microlens is planar, while the other is curved. Refractive microlenses and microlens arrays are available in a variety of organic and inorganic materials, including resins such as photoresist, dielectric materials such as glasses, and semiconductors such as silicon and GaAs. The curvature can be obtained by a wide variety of techniques.
The prior art describes several techniques that produce high-quality diffraction-limited refractive microlenses. Overview articles have been published by P. Pantelis et al., "Polymer microlens arrays", Pure Appl. Opt. 3, pp. 103-108 (1994), and by D. R. Purdy, "Fabrication of complex micro-optic components using photo-sculpting through halftone transmission masks", Pure Appl. Opt. 3, pp. 167-175, (1994).
A particular class of techniques concentrates on forming microlenses in thermoplastic resins like photoresist. An example is published by Popovic et al. in the reference SPIE 898, pp.23-25 (1988). The technique, named reflow technique, comprises the steps of defining the lenses' footprint in a thermoplastic resin, e.g. by photolithography in a photosensitive resin like photoresist, and subsequently heating this material above its reflow temperature. The surface tension draws the island of photoresist into a spherical cap with a volume equal to the original island before reflow. This cap is a plano-convex microlens. Advantages of the technique are, amongst others, the simplicity, the reproducibility, the possibility of integration directly on top of a light-emitting or light-detecting optoelectronic device. The maximum achievable diameter of such lens is limited to approximately 700 .mu.m: a larger island of photoresist does not reflow into a spherical cap but sags in the middle.
Other prior art techniques improve the mechanical strength and chemical stability of photoresist lenses, which improve their market value. One possibility is to transfer the lens shape into the substrate material by etching. Another solution is the use of negative photoresists instead of positive photoresists, as they are chemically more stable. Yet another possibility is implantation of Si to bring the resin material to cross-linking to become insoluble.
A drawback of said reflow technique is that the portion of the sphere which is obtained is always substantially smaller than a hemisphere. In other words, the cap height is much smaller than the radius of curvature. This is due to the limited aspect ratio of the photoresist island before reflow. The limited ratio of peak height to radius of the lens means that such lens is not suited for improving the light output of solid-state light-emitters like planar light-emitting diodes which have a Lambertian emission profile (i.e., they emit light in all directions). Indeed, the optimum lens shape for this purpose is an almost complete sphere, or, even better, an ellipsoid with the long axis perpendicular to the light-emitting surface. Such shapes cannot be made with prior-art photoresist reflow techniques.
In U.S. Pat. No. 4,689,291 a method has been proposed to improve the repeatability of the reflow technique by confining the reflow to a pedestal formed prior to the microlens fabrication. This method is also described in above-mentioned reference Popovic et al., SPIE 898, PP. 23-25 (1988). Lenses formed according to this method have an improved ratio of peak height to radius, and hence are better suited for integration on light emitters. As shown in FIG. 2 of said reference, the shape of the lens obtained in this way is, however, still far from hemispherical.